As is known, jet aircraft have turbine engine(s) which provide propulsion power to maintain flight velocity and generate on board power which includes electrical and hydraulic power for controlling the control surfaces and the electrical loads of the aircraft.
For various reasons jet aircraft turbine engines are subject to engine failures which may occur at any time. The engine failures may occur at low altitudes where air for the combustion process is plentiful, or at high altitudes where air for the combustion process is not available in great quantities. Emergency power units (EPU's) provide necessary hydraulic and electrical power when all of the aircraft turbine engines fail and the air breathing auxiliary power unit, which is a conventional air aspirated turbine used for providing ground power for starting, etc., and may be used for providing in flight hydraulic and electrical power, cannot provide sufficient power to provide the necessary electrical and hydraulic power to maintain control of the flight surfaces. EPU's are designed to operate from stored fuel and oxidizer at all altitudes, in order to generate emergency on board electrical and hydraulic power during engine failures. The emergency on board electrical and hydraulic power permits the pilot of the aircraft to manipulate the control surfaces and electrical loads until the engines are restarted or until the aircraft can safely be landed.
EPU's are disclosed by U.S. Pat. Nos. 3,722,217; 3,800,534 and 4,033,115. The EPU's disclosed in the above listed patents generally create hot gases to drive the blades of a turbine by the decomposition of a fuel which supplies its own oxidizer, thus not having an oxidizing gas bottle.
Other conventional systems cause combustion of an air-fuel mixture with the air being supplied by an on-board air bottle with the fuel being a fuel such as JP4.
The conventional systems which supply air for combustion using an air bottle suffer from various disadvantages. Namely, when air from the air bottle is used, it is expanded (blown down) over a period of operation but still has to provide a usable pressure which is designed to achieve full output power that the EPU is designed to provide to satisfy potential peak hydraulic and electrical power demands. The expansion of the air from the air bottle causes the air to become cooler in the air bottle which has the effect of further reducing the available pressure and to supply cool air to the combustor.
The cool air being supplied to the combustor causes low temperature problems when mixed with liquid fuel in the combustor. The cool air in the bottle is reduced in pressure due to the contraction of the air which prevents a significant amount of the stored air from being used during operation of the EPU. Therefore, the full charge of air which is available at normal operating temperatures during flight is never used as a consequence of the foregoing cooling which occurs during operation of the EPU. As a consequence, the size of the air bottle must be proportionally increased to satisfy the requirement for cooled stored air which must be delivered during operation that is necessary to satisfy power requirements for the EPU. A weight penalty of 20 or more pounds is required for a typical system to provide the additional volume of the gas bottle to satisfy the EPU power requirements. Accordingly, there is a need for an emergency power unit which overcomes the above described disadvantages.